Description (taken from application): To provide a common facility for radioactive labeling of hormones, proteins, antibodies and other material with 125-I or 131-I. Specifically, this means that the Core runs a "hot lab" with an iodination hood and dedicated equipment for protein purification. All procedures are done by the technician or the Core Director. We have found several advantages to this method. The radioactive materials produced by the Core are uniform and routinely standardized for quality before being released to investigators, allowing comparison of results among a series of experiments done in the same laboratory, in different laboratories, or as part of a collaborative study. The efficiency and expertise of the laboratory as well as the bulk production of routinely labeled compounds has also resulted in substantial savings (versus commercially purchased materials), safety (compared with multiple small iodination laboratories across campus) and the ready availability of labeled reagents that are either not available commercially or are too costly for routine use by investigators.